Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a polarizer and analyzer. Incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. The contrast of an LCD is also dependent on the angle from which the display screen is viewed. One of the common methods to improve the viewing angle characteristic of LCDs is to use compensation films. Birefringence dispersion is an essential property in many optical components such as compensation films used to improve the liquid crystal display image quality. Even with a compensation film, the dark state can have undesirable color tint such as red or blue, if the birefringence dispersion of the compensation film is not optimized.
A material that displays at least two different indices of refraction is said to be birefringent. In general, birefringent media are characterized by three indices of refraction, nx, ny, and nz. The out-of-plane birefringence is usually defined by Δnth=nz−(nx+ny)/2, where nx, ny, and nz are indices in the x, y, and z direction, respectively. Indices of refraction are functions of wavelength (λ). Accordingly, out-of-plane birefringence, given by Δnth=nz−(nx+ny)/2 also depends on λ. Such a dependence of birefringence on λ is typically called birefringence dispersion.
Birefringence dispersion is an essential property in many optical components such as compensation films used to improve the liquid crystal display image quality. If nx=ny, the media is called C-plate and Δnth=nz−nx, or equivalently Δnth=nz−ny.
Adjusting Δnth dispersion, along with in-plane birefringence (nx−ny) dispersion, is critical for optimizing the performance of optical components such as compensation films. The Δnth can be negative (102) or positive (104) throughout the wavelength of interest, as shown in FIG. 1. In most cases, film made by casting polymer having positive intrinsic birefringence, Δnint, gives negative Δnth. Its dispersion is such that the Δnth value becomes less negative at longer wavelength (102). On the other hand, by casting polymer with negative Δnint, one obtains a positive Δnth value with less positive Δnth value at longer wavelength (104). The dispersion behavior, in which the absolute value of Δnth decreases with increasing wavelength, is called “normal” dispersion. In contrast to normal dispersion, it is often desirable to have Δnth essentially constant over visible wavelength λ (between 400 nm and 650 nm) (curves 106 and 108 in FIG. 1). Hereinafter, the term “essentially constant” means that for at any two wavelengths λ4≈λ5 such that 400 nm<λ4, λ5<650 nm, we have 0.95<|Δnth(λ4)|/|Δnth(λ5)|<1.050. Particularly useful media are ones having low and constant Δnth satisfying |Δnth(λ)|<0.0001 for wavelength λ satisfying 400 nm<λ<650 nm (curve 110 in FIG. 1). Thus, such media exhibit essentially zero birefringence.
In still other cases, it is desirable for the absolute value of Δnth to increase at longer wavelength. Such behavior is called “reverse” dispersion (curves 202, 204 in FIG. 2).
These cases of different behaviors in Δnth in principle can be achieved by suitable combination of two or more layers having difference dispersion in Δnth. Such an approach, however, is difficult, as one has to carefully adjust the thickness of each layer. Also, extra process steps are added to manufacturing.
U.S. Pat. No. 6,565,974 discloses controlling birefringence dispersion by means of balancing the optical anisotropy of the main chain and side chain group of a polymer. This method enables the generation of a polymer having smaller birefringence (or equivalent retardation value) at shorter wavelength, a reverse dispersion material. Polymeric materials are flexible and easy to process. However, the chemical structure of the polymer, which is mainly composed of carbon hydrogen, limits the range of birefringence behavior including dispersion. This makes control of birefringent dispersion difficult, even by mixing two polymers, co-polymerization, and other possible methods. Thus, a polymeric entity alone has only a limited capability for controlling birefringent behavior.
Inorganic materials have various intrinsic birefringence behaviors. Some inorganic materials show positive while others exhibit negative intrinsic birefringence. The Table below shows the intrinsic birefringence at λ=632 nm of various inorganic materials. These are uniaxial materials and, thus, have extraordinary (nc) and ordinary (no) indices of refraction wherein the intrinsic birefringence Δnint is defined as Δnint=ne−no.
TABLE 1CalciteCaCO3Δnint = −0.154MagneciteMgCO3Δnint = −0.192GeikieliteMgTiO3Δnint = −0.360RutileTiO2Δnint = +0.287CassiteriteSnO2Δnint = +0.097
As the above Table shows, the inorganic materials exhibit a wide range of birefringence magnitude. They also have various dispersion behaviors and, thus, they are more versatile than polymeric materials for optical applications.
Although inorganic materials offer versatile birefringence properties, they are costly and difficult to process. In order to utilize their birefringent behavior, however, the inorganic materials have to be single crystalline of appreciative size, or else one sees only averaged isotropic effects.
It would be very much desired to discover a material that combines the processability of polymers and the versatility of inorganic materials with respect to Δnth dispersion control, in order to provide more latitude than combinations of polymers. It would be especially desirable to be able to easily make such materials into films that can be used as compensation films for LCDs.
Many efforts have been made to make organic-inorganic hybrid materials. One method is to blend inorganic nanoparticles with a binder. Unfortunately, the undesirable aggregation of the nanoparticles in such materials when made into films can result in non-transparency. Such films are not useful for optical applications which require high transparency and low haze.
U.S. Pat. No. 6,599,631 and U.S. Pat. No. 6,656,990 describe blending polymer and inorganic particles to form hybrid materials. Both patents require specially prepared particles with well-controlled particle size and surface treatment of the particles. These very specific requirements make these methods of forming hybrid materials unattractive as a low-cost process.
U.S. Pat. No. 6,586,515 to Koike disclose non-birefringent optical films made from a nanocomposite in which a fine inorganic substance is oriented in the same direction as the linked chains of a resin oriented under an external force, wherein the birefringence of the inorganic substance cancels out the birefringence of the resin. To solve the problem of dispersibility, the inorganic substance is subjected to a surface treatment for dispersion in the resin prior to kneading the inorganic substance into the resin material. To develop effective birefringence, the inorganic substance comprise particles having an elongated shape, including acicular, cylindrical, plate, columnar, and ellipsoid shape.